IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
Figure 4.6. The IRAC point res ponse functions (PRFs) at 3.6, 4.5, 5.8 and 8.0 microns. The PRFs were
generated from models refined with in-flight calibration test data invol vi ng a bright cali brati on star observed
at several epochs. Central PRFs for each channel are shown above wi th a logarithmic scaling to hel p dis play
the entire dynamic range. The PRFs are shown as they appear with 1/5th the native IRAC pixel sampling of
1.2 arcseconds to highlight the core structure.
4.7.2 Extended PRFs
The FITS files of the extended PRFs can be obtained using the links in the IRAC web pages. In order to
gain high signal-to-noise out to the edge of the arrays, PRFs were generated from a combination of onboard
calibration and science observations of stars with different brightness, joined together to produce
extended high dynamic range (HDR) observational PRFs. These PRFs have two main components: a core
HDR PRF created by the observations of a reference star, and the extended region from observations of a
set of bright stars that saturated the IRAC array. They can be used to perform source extraction and PRFfitting
photometry of bright, highly saturated stars with extended wings. The core of the extended PRF
was generated using the prf_estimate module of MOPEX which has been shown to be inadequate for
making high-quality PRFs for IRAC. As a result, the extended PRF should not be used for PRF-fitting
photometry and source extraction of non-saturated point sources. Instead, the core PRF in Section 4.7.1 is
more appropriate for PRF-fitting photometry. Also, note that the detailed structure of the center of
saturated sources fitted using the extended PRF will not be correct in detail.
These extended HDR PRFs have a pixel size of 0.2 IRAC pixels, or ~ 0.24 arcsec. The size of each PRF
image is 1281x1281 pixels, covering an area of ~ 5.1 arcmin x 5.1 arcmin. The PRFs are centered within
each image. The PRFs are calibrated in MJy/sr. The PRFs represent an unsaturated, very high S/N image
of Vega, and the flux density contained within a 10 native IRAC pixel aperture radius (50 HDR PRF
pixels), with the sky level estimated in a radial annulus from 12 to 20 native IRAC pixels, is equal to the
flux density of Vega. The pedestal level of each image is set to zero in the corners of each PRF.
To produce the core portion of the HDR PRF, 300 HDR observations of a calibration star were obtained
during three separate epochs, each observation consisting of short exposures (0.6 sec/1.2 sec) and long
exposures (12 sec/30 sec). The HDR PRFs were generated by first combining short-exposure frames and
long-exposure frames separately. The short frames enabled the cores to be constructed without a
saturation problem, while the long exposures allowed the construction of a higher signal-to-noise PRF in
the wings out to 15 arcseconds. The assembly required the replacement of any saturated areas in the longexposure
frames with unsaturated data from the same pixel area of the short-exposure frames. It also
required the replacement of a few pixels in the long-exposure frames by the corresponding pixels in the
short-exposure frames to mitigate the non-linear bandwidth effect in channels 3 and 4. The "stitching" of
the two components of the HDR PRF was completed using a 1/r masking algorithm requiring a
percentage of each frame to be added together over a small annulus two IRAC pixels in width just outside
the saturated area. Each epoch was treated separately and then all three epochs were aligned and a median
was taken to remove background stars.
Calibration 49 IRAC Point Spread and Point
Response Functions